Interstellar objects: ʻOumuamua, Borisov and 3I/ATLAS

At a glance

Interstellar objects are asteroids and comets that formed around other stars and are now passing through the Solar System on orbits that are not bound to the Sun. Only three have been identified so far: 1I/ʻOumuamua in 2017, 2I/Borisov in 2019 and 3I/ATLAS in 2025. The “I” in their designations stands for interstellar; the number gives the order of discovery. Each is a sample of material from another planetary system that we can study without leaving home.

Sources and dates: orbital elements from the JPL Small-Body Database (3I/ATLAS solution dated 19 February 2026) and Seligman et al. (2025); discovery and size data from NASA pages for each object (3I/ATLAS page updated 28 May 2026). v∞ for 1I is NASA’s figure; for 2I it was computed by SpaceTRC from JPL’s e and q (v∞ = √(GM☉(e − 1)/q)); for 3I it is from Seligman et al.

Sources: [1] [2] [3] [4] [6]

What makes an object interstellar?

Every body orbiting the Sun follows a conic section (see our orbital mechanics guide). Planets, asteroids and most comets move on closed ellipses, with eccentricity e below 1. An object arriving from interstellar space, by contrast, has more energy than the Sun’s gravity can hold: it follows an open hyperbola with e greater than 1, swings once around the Sun and leaves for ever. The parabola, with e exactly 1, is the dividing line.

The key physical quantity is the hyperbolic excess speed, v∞: the speed the object would have very far from the Sun. As it falls inward it speeds up, reaches its maximum speed at perihelion and slows down again on the way out, returning to v∞. Eccentricity alone can mislead, because long-period comets from the Oort cloud sometimes have e only slightly above 1 after a nudge from the planets. What marks a true interstellar object is a clearly hyperbolic orbit with a large v∞, of tens of kilometres per second, which matches the typical relative motions of stars in the Sun’s neighbourhood.

3I/ATLAS illustrates the point. With e ≈ 6.14 and v∞ ≈ 58 km/s, its orbit is the most hyperbolic yet measured. It also moves almost exactly in the plane of the planets but in the opposite direction: its inclination of about 175° makes its orbit retrograde.

Why should such objects exist at all? Models of planet formation predict that growing giant planets fling vast numbers of leftover planetesimals out of their systems, and our own Solar System is thought to have lost material in the same way (see formation of the Solar System). The Galaxy should therefore be filled with drifting comets and asteroids. Detecting them is hard because they are small, faint and fast, and they are usually found only when they come close to the Sun and, for comets, start to release gas and dust.

Sources: [2] [3]

Interactive lab: a hyperbolic passage

Enter an approach speed v∞ and a perihelion distance q, or choose one of the three known interstellar objects. The calculator returns the orbit’s eccentricity, the speed at perihelion, the Sun’s escape speed at that distance and the angle through which the Sun bends the path.

Compare the presets. ʻOumuamua passed close to the Sun with a modest v∞, so its path was bent strongly; 3I/ATLAS arrived so fast that the Sun deflected it by only about 19°. For 3I the computed perihelion speed, about 68 km/s, matches NASA’s published figure of roughly 246,000 km/h. As the notes under the calculator explain, it ignores the planets’ pull and the jet-like push of cometary outgassing.

1I/ʻOumuamua: the first, and the strangest

On 19 October 2017 the Pan-STARRS1 telescope in Hawaiʻi, funded by NASA’s Near-Earth Object Observations programme, spotted a faint object already moving away from the Sun. It had passed perihelion on 9 September, only about a quarter of an astronomical unit from the Sun. Its name is Hawaiian for “a messenger from afar arriving first”.

ʻOumuamua was unlike any known asteroid or comet. Its brightness varied by a factor of ten as it spun every 7.3 hours, implying an extremely elongated shape, perhaps ten times longer than wide and up to about 400 m long. Deep images showed no coma of gas or dust. Yet in 2018 Marco Micheli and colleagues reported in Nature that its trajectory deviated from a purely gravitational path, with a non-gravitational acceleration detected at about 30σ significance, directed away from the Sun and weakening with distance.

The team examined radiation pressure, drag, solar-wind effects and shape-related artefacts and argued that comet-like outgassing was a physically viable explanation, provided the gas left too little dust to be seen. Other explanations have since been proposed, and the debate is not settled; with the object long gone, no new data can be collected.

Sources: [4] [5]

2I/Borisov: an unmistakable comet

The second visitor was found on 30 August 2019 by Gennady Borisov, an amateur astronomer using a telescope he built himself. Unlike ʻOumuamua, 2I/Borisov was discovered on its way in and looked like an ordinary comet, with a coma and tail. That made it possible to study its chemistry in detail while it passed perihelion in December 2019.

Hubble observations led by Dennis Bodewits and ALMA observations led by Martin Cordiner and Stefanie Milam, published in Nature Astronomy in 2020, showed that Borisov released unusually large amounts of carbon monoxide relative to water: a higher concentration than in any comet previously observed within about 300 million km of the Sun. Carbon monoxide ice survives only at very low temperatures, so Borisov probably formed in a very cold environment, such as the outer regions of another star’s planet-forming disc, although its exact birthplace cannot be traced.

Sources: [6]

3I/ATLAS: the fastest visitor

The NASA-funded ATLAS survey telescope at Río Hurtado in Chile reported the third interstellar object to the Minor Planet Center on 1 July 2025, when it was about 670 million km from the Sun, inside the orbit of Jupiter. It also received the comet designation C/2025 N1 (ATLAS). Within days its orbit was shown to be strongly hyperbolic, and the discovery paper by Darryl Seligman and colleagues gave e ≈ 6.1, v∞ ≈ 58 km/s, q ≈ 1.36 au and an inclination of about 175°.

The comet was active from the start. Hubble images on 20 August 2025 constrained the solid nucleus to between about 440 m and 5.6 km across. JWST spectra taken at 3.32 au from the Sun, reported by Cordiner and colleagues, revealed a coma dominated by carbon dioxide, with water, carbon monoxide, carbonyl sulphide, water ice and dust. Its CO₂-to-water ratio of 7.6 ± 0.3 is among the highest ever measured in a comet. The authors suggest that the nucleus may be intrinsically CO₂-rich, perhaps because its ices were exposed to more radiation than Solar System comets or because it formed near a CO₂ ice line.

3I/ATLAS reached perihelion at about 1.36 au in late October 2025, just inside the orbit of Mars, while it was on the far side of the Sun as seen from Earth. Its closest approach to Earth, about 1.8 au, came around 19 December 2025, and it passed Jupiter in March 2026 on its way out of the Solar System.

Sources: [1] [2] [7]

A Solar-System-wide observing campaign

Because 3I/ATLAS was hidden behind the Sun from Earth near perihelion, spacecraft elsewhere in the Solar System provided unique viewpoints. On 3 October 2025 the comet passed about 30 million km from Mars. ESA’s ExoMars Trace Gas Orbiter and Mars Express observed it between 1 and 7 October, and NASA lists MRO, MAVEN and the Perseverance rover among its Mars assets that took part.

ESA’s Juice, on its way to Jupiter, observed the comet from 2 to 25 November 2025, just after perihelion when it was most active, with its closest approach on 4 November at about 0.4 au (60 million km). Five instruments (JANUS, MAJIS, UVS, SWI and PEP) took part; the data were stored on board and downlinked on 17 and 20 February 2026. NASA’s Europa Clipper pointed its ultraviolet spectrograph, Europa-UVS, at the comet for about seven hours on 6 November 2025 from about 164 million km, capturing its gas and dust comae and its tails.

Many other observatories joined in: NASA’s list of missions that observed the comet includes Hubble, Webb, SPHEREx, Parker Solar Probe, Psyche, Lucy, PUNCH, SOHO and STEREO.

Sources: [1] [8] [9] [10]

Interstellar meteors: a contested claim

If interstellar objects pass through the Solar System, some small ones should occasionally hit Earth’s atmosphere. In 2019 Amir Siraj and Abraham Loeb argued that a fireball recorded by US government sensors on 8 January 2014, whose data are published through NASA’s Center for Near Earth Object Studies (CNEOS), had arrived on a hyperbolic orbit and was therefore interstellar. The claim relied on the speed reported for the event.

Many meteor specialists are sceptical. Peter Brown and Jiří Borovička, writing in The Astrophysical Journal in 2023, compared US government sensor data with independent measurements of other fireballs and found that speed estimates at high velocities can be off by 10–15 km/s and directions by more than 10° on average. They concluded that the simplest explanation for the 2014 event is a substantially overestimated speed, which would remove the need for an interstellar origin. No interstellar meteor is currently regarded as confirmed.

Sources: [13]

What comes next: Rubin and Comet Interceptor

The discovery rate should soon rise. The NSF–DOE Vera C. Rubin Observatory in Chile, whose first images were released in June 2025, officially began its ten-year Legacy Survey of Space and Time in 2026, with the start announced on 30 June 2026; in early survey operations it had already found more than 11,000 new asteroids in about six weeks. A 2021 study by Devin Hoover, Darryl Seligman and Matthew Payne estimated that the survey should detect about 15 interstellar objects over ten years, and described the figure as a lower limit pending better knowledge of their numbers and sizes.

Reaching one would be harder. ESA’s Comet Interceptor, led with JAXA and planned for launch in late 2028 or early 2029 from Europe’s Spaceport in French Guiana, will consist of three spacecraft carrying ten instruments. It will be parked in space until a suitable pristine comet or interstellar object is found, then sent to fly past it. The Hoover study warned that the chance of an interstellar object being reachable with a mission of Comet Interceptor’s capability is very small, so its most likely target is a pristine comet arriving from the outer reaches of the Solar System.

Sources: [11] [12] [14]

Current research (2024–2026)

• 3I/ATLAS discovered (July 2025). The third interstellar object, whose coma JWST found to be dominated by CO₂; its orbit (JPL solution of 19 February 2026) has e ≈ 6.14.
• Observed from Mars and deep space (October–November 2025). Mars orbiters, Juice and Europa Clipper observed the comet near perihelion; Juice’s data reached Earth in February 2026 and were being analysed in March 2026, according to ESA.
• Rubin survey under way (June 2026). The Legacy Survey of Space and Time has officially started and is expected to multiply the number of known interstellar objects.
• Comet Interceptor (launch late 2028–early 2029). The mission is designed to fly past a pristine comet or, if one proves reachable, an interstellar object.

Sources: [3] [7] [9] [12] [14]

Known vs open questions

Well established:

• Three objects with clearly hyperbolic orbits and large v∞ have passed through the Solar System, proving that other planetary systems eject small bodies.
• 2I/Borisov and 3I/ATLAS are comets whose compositions (CO-rich and CO₂-rich) differ from typical Solar System comets.
• ʻOumuamua showed a non-gravitational acceleration without a visible coma.

Still open:

• What caused ʻOumuamua’s acceleration, and what it was made of.
• Which kinds of stars and systems these objects came from, and how old they are.
• How many interstellar objects exist and what their size distribution is; Rubin should answer this statistically.
• Whether any interstellar meteor has ever been recorded in Earth’s atmosphere.

How to go further: data and tools

Astrometric observations and orbits of all three objects are maintained by the IAU Minor Planet Center; the JPL Small-Body Database gives orbital elements with uncertainties (search for 1I, 2I or 3I). Spacecraft observations of comets are archived at NASA’s PDS Small Bodies Node. For the environment these objects travel through between the stars, see our research dossier on the interstellar medium.

Related on this site

• Research dossier: The interstellar medium
• Small bodies: asteroids and comets
• Formation of the Solar System
• Stars and stellar evolution
• Europa Clipper
• Space guide: Orbital mechanics
• Orbit lab